The present invention relates to reducing the amount of ether content in polyesters as a by-product.
Heretofore, polyesters have been prepared by the reaction of a carboxylic acid or esters thereof with polyols, e.g., glycols. As well known to those skilled in the art, an ether by-product is produced during the process, and therefore is inherently contained in the polyester. Ether links are light sensitive and thus degrade in the presence of light. Moreover, in the presence of heat, thermal degradation readily occurs. The net result is a polyester having reduced stability such that the molecular weight is affected by light or heat and reduces various physical properties such as tensile strength, flex strength, and the like.
Heretofore, U.S. Pat. No. 3,442,868 taught a process for producing fiber-forming polyesters, for example, polyethylene terephthalate without direct contact of the acid and glycol components comprising, as a first step, reacting terephthalic acid with an oligomer (A) comprising the reaction product of such terephthalic acid and a glycol, said oligomer (A) having a degree of polymerization of one or more but less than three and having terminal hydroxyl groups to form an oligomer (B) having a higher degree of polymerization, that is from 3 to 10 and hydroxyl groups substantially at the termini; as a second step, reacting at least a portion of an oligomer (B) with a glycol to depolymerize (B) to form oligomer (A') having a degree of polymerization of one or more but less than three; and, as a third step, forming the fiber-forming polyester by polycondensing a material comprising any remaining portion of an oligomer (B), a portion of oligomer (A') or mixtures thereof while recycling the remaining portion of oligomer (A') for use as oligomer (A) in the first step. Thus, such a process basically relates to a glycolizing step which actually breaks down the oligomer to produce bis(2-hydroxyethyl) terephthalate. Thus, ethylene glycol is added to actually break down the chain. Moreover, large amounts of ethylene glycol are used initially.
U.S. Pat. No. 3,849,379 is similar in that an amount of precondensate is glycolized to bis(2-hydroxyethyl) terephthalate by the addition of ethylene glycol. Hence, it is not pertinent.
Other patents relating to the production of polyesters, largely with regard to producing low glycol ether content include the following: U.S. Pat. No. 4,079,046; 4,077,945; 3,919,288; 3,819,585; 3,697,579; 3,651,125; 3,644,483; 3,534,082; 3,506,622; 3,487,049; and 3,468,849. However, none of these patents relate to applicant's incremental addition of initially a majority of carboxylic acids followed by a later stage addition of a majority of a glycol.